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Oligohydramnios

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The volume is determined by a steady state between input: fetal urination 400-1000 ml/da, alveolar exudate 600/900 ml/day, and outflow: fetal swallowing 250-450 ml/day and reabsorption through the chorioamniotic membranes 80 ml/day. Input is affected by maternal blood flow to the placenta and the capacity for fetal maternal exchange at the intervillous space, modulated by maternal serum osmolality and maternal intervascular volume. Oligohydramnios is associated with increased perinatal morbidity and mortality and develops in 0.5-4.0 percent of all pregnancies. It has been associated with congenital anomalies such as renal dysgenesis or obstruction, intrauterine growth retardation (IUGR), post maturity and placental problems related to maternal fetal interaction. When oligohydramnios develops early in pregnancy, the fetal outcome is generally poor due to development of pulmonary hypoplasia, fetal compression syndrome, and possible amniotic band syndrome. Barss et al. found second trimester oligohydramnios to be associated with no survivors in their series of twelve pregnancies. Without adequate lung development, extrauterine life is impossible. Premature rupture of membranes is a cause of oligohydramnios and occurs in approximately 10 percent of all pregnancies. It has been correlated with an increased incidence of perinatal death, fetal distress in labour, and infection. Oligohydramnios associated with IUGR may result from intrauterine hypoxemia which causes shunting of fetal blood flow away from the kidneys and lungs to more vital organs, the brain and heart. Decreased urine production and alveolar transudate result in a decrease of amniotic fluid production. Maternal hypertension may result in IUGR from placental insufficiency. When hypertension is a manifestation of pre-eclampsia, reduction of the maternal intravascular volume may also be reflected by fetal volume contraction with resultant fetal oliguria and oligohydramnios. Certain maternal drug therapies such as maternal indomethacin treatment, often used as an adjunct in treatment of premature labour may induce oligohydramnios. The mechanism is thought to relate to decreased renal perfusion.Oligohydramnios is diagnosed by subjective assessment by an experienced examiner of a markedly decreased or absent amniotic fluid volume. A largest pocket with a vertical diameter of less than 1 cm or a diameter of 1 cm or less in two perpendicular planes is consistent with severe oligohydramnios. An AFI below the 5th percentile for gestational age also indicates oligohydramnios, and an AFI less than 5.1 cm is also considered consistent with oligohydramnios. When oligohydramnios is diagnosed, careful assessment for fetal malformations is indicated including observation of fetal urinary bladder filling. Unfortunately, the presence of oligohydramnios makes such a search more difficult. Colour flow doppler studies, amnioinfusion, and the transvaginal rather than the transabdominal approach may be helpful in selected cases. Title: In: Ultrasound in Obstetrics and Gynecology Vol I Author: Chervenak FA, Isaacson GC, Campbell S Journal: Little, Brown, & Co, p555-563, p565-568 Year: 1993 Title: In: Ultrasound in Obstetrics and Gynecology Vol I Author: Chervenak FA, Isaacson GC, Campbell S Journal: Little, Brown, & Co, p1063-1081 Year: 1993 Title: In: Diagnostic Ultrasound of Fetal Anomalies Vol II Author: Nyberg D, Mahony B, Pretorius D Journal: Mosby Year Book: St. Louis, p38-50 Year: 1990 Title: In: The Principles and Practice of Ultrasonography and Obstetrics and Gynecology, 4th Edition Author: Fleischer A, Romero R, Manning F, Jeanty P, James Jr. A Journal: Appleton & Lange: Norwalk, CT, p196 Year: 1991 Title: Potter's syndrome in the 2nd trimester - prenatal screening and pathological findings in 60 cases of oligohydramnios sequence Author: Scott RJ, Goodvurn SF Journal: Prenatal Diag 15:519-29 Year: 1995 Title: Oligohydramnios sequence: spectrum of renal malformations Author: Newboald MJ, Barson AJ Journal: Br J Obstet Gynecol Vol 101:598-604 Year: 1994 Title: Second trimester oligohydramnios. A predictor of poor fetal outcome Author: Barss VA, Benacerraf BR, Frigoletto FD Journal: Obstet Gynecol 64:608 Year: 1984

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